Many processes for the removal of volatile solvents from bulk masses are known. Air drying is often unsatisfactory for various reasons. As solvent poor dry air passes over a solvent rich area, a fraction of the solvent evaporates when the equilibrium is established. As the solvent enriched air is removed (by normal air currents or otherwise), it is replaced with additional solvent poor air and more solvent is given up. The process continues until equilibrium is reached and all or most of the solvent has been removed. Under certain conditions (i.e. high solvent volatility, minimal solvent/substrate affinity, high diffusion rates, etc.) such a process may take place rapidly within minutes, while in other cases this solvent loss is inhibited due to low solvent volatility, strong solvent/substrate binding, or low diffusion rates. In many cases the evaporation rate is unacceptably low.
Many alternative processes and improvements are known. For example, heat is applied to accelerate the evaporation rate, pressure reductions are applied as well as combination thereof. Liquid solvent exchange (leaching) is also used. In this process the solid material containing the residual solvent is exposed to another liquid which has less solubility for the solid material from which the residual solvent is to be removed, but a greater solubility for the residual solvent. The residual solvent is then transferred from the solid material to the other liquid and the liquid enriched in solvent is then separated from the solid material.
The removal of solvent can be extremely difficult from a formed compressed article such as uncoated or coated tablets. The removal from coated tablets requires the complete evaporation from the film coating and the covered compressed tablet core.
The increasing demand for extremely low solvent content in pharmaceutical dosage forms such as coated tablets may mandate that none of the foregoing conventional solvent removal processes is satisfactory.
The extraction of residual solvents by exposure of the article to be made solvent free to an inert phase of a fluid under supercritical conditions or to an inert phase of a gas under near-critical conditions is another known process that has been applied to solids such as bulk masses. Especially in the food industry this method has been applied to remove caffeine from green coffee or nicotine and liquid admixtures from tobacco. This method is applicable wherever esthetic qualities associated with the solvent free article, such as shape, color, surface characteristics or physical appearance are of little concern.
It has not been attempted before to expose solid articles such as formed compressed tablets, which also contain residual solvent, to fluids under supercritical conditions or gases under near-critical conditions.